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Abstract:

There is disclosed a device and method for the generation of zero
emission electricity that can be used to provide load balancing and
emergency support to a electricity distribution network or back up
electricity to a critical consumer such as a hospital or data centre. The
system uses a cryogenic fluid and a source of low grade waste heat. A
cryogenic fluid is first evaporated by an evaporator (3) heated by a
superheater (4) before entering an expansion turbine (10) to produce
electricity

Claims:

1. An electricity generation device comprising: a storage tank for
storing a cryogenic fluid, a fluid pump for compressing cryogenic fluid
taken from the storage tank to a high pressure, an evaporator for
evaporating the high pressure cryogenic fluid, to provide a high pressure
gas, a superheater for heating the high pressure gas to a high
temperature using a source of heat from a co-located process, and an
expansion turbine for expanding and extracting work from the superheated
high pressure gas and for driving a generator to produce electricity from
the rotational energy produced by the expansion turbine, wherein the
evaporator is for evaporating the high pressure cryogenic fluid using the
low pressure exhaust from the expansion turbine.

2. The electricity generation device of claim 1, wherein the device is
connected to an electricity generation network to provide at least one
grid support service.

3. The electricity generating device of claim 1, further comprising a
main heater, wherein the main heater and the superheater are arranged to
heat the high pressure gas from the evaporator in two stages using at
least one source of heat from at least one co-located process.

4. The electricity generating device of claim 3, wherein a first heat
transfer media is arranged to transfer heat from the at least one source
of heat to the main heater, and a second heat transfer media different
from the first heat transfer media is arranged to transfer heat from the
at least one source of heat to the superheater.

5. The electricity generating device of claim 1, wherein the expansion
turbine comprises a multi stage turbine, and further comprising a
re-heater arranged between each stage of the multi-stage turbine to heat
the cooled exhaust from the previous stage of the turbine before the
exhaust gas enters the next turbine stage.

6. The electricity generation device of claim 1, wherein a final exhaust
emitted of the evaporator is arranged to provide cold energy for use in a
co-located process.

7. A method of generating electricity comprising: storing a cryogenic
fluid in a storage tank; extracting the cryogenic fluid from the storage
tank and compressing the cryogenic fluid to a high pressure using a fluid
pump; evaporating the high pressure cryogenic fluid in an evaporator
using the low pressure exhaust of an expansion turbine to provide a high
pressure gas; heating the high pressure gas from the evaporator to a high
temperature using a superheater and a source of heat from a co-located
process; expanding the superheated high pressure gas using the expansion
turbine; and extracting work from the high pressure gas to drive a
generator and produce electricity from the rotational energy produced by
the expansion turbine.

8. The method of claim 7, wherein the co-located process is a power
station or an industrial process.

9. The method of claim 7, wherein the cryogenic fluid is liquid nitrogen
or liquid air.

10. The method of claim 7, further comprising: supplying the electricity
produced by the generator to the electricity distribution grid to provide
at least one grid support service.

11. The method of claim 10 wherein the at least one grid support service
comprises at least one of: a) balancing differences in supply and demand
at different times of day; b) balancing differences in supply and demand
at short notice; c) injecting electricity into the grid to support
frequency when demand is increasing rapidly; d) providing black start
support; and e) providing electricity distribution grid re-enforcement
when parts of the electricity distribution grid have a shortfall in
capacity during periods of high power demand.

12. The method of claims 7, further comprising: using the generated
electricity to provide back-up power.

13. The method of claims 7, wherein the step of heating the high pressure
gas comprises: heating the high pressure gas during a first stage using a
main heater using energy from at least one co-located process; and
heating the high pressure gas during a second stage using a superheater
using energy from at least one co-located process.

14. The method of claim 13, wherein energy is transferred to the main
heater during the first stage using a first heat transfer media, and
energy is transferred to the superheater during the second stage using a
second heat transfer media different from the first heat transfer media.

15. The method of claims 7, wherein the step of expanding the superheated
high pressure gas comprises expanding the gas in a multi-stage turbine
by: expanding the gas in a first stage of the multi-stage turbine;
heating the exhaust gas from the first stage of the multi-stage turbine
with a re-heater; and expanding the exhaust gas from the re-heater in a
second stage of the multi-stage turbine.

16. The method of claims 7, further comprising extracting cold energy
from a final exhaust of the evaporator, and using the extracted cold
energy in a co-located process.

17. (canceled)

18. The electricity generation device of claim 1, wherein the co-located
process is a power station or an industrial process.

Description:

[0001] The present invention relates to electricity generation devices and
methods that use a cryogenic fluid such as liquid nitrogen or liquid air
and a source of low grade waste heat.

[0002] BACKGROUND OF THE INVENTION

[0003] Electricity distribution networks (or grids) are often supported by
a fleet of diesel generators and open cycle gas turbines that provide
electricity during periods of high demand and emergency events such as
the unexpected failure of a power station. Such generating assets, often
referred to as peaking plant, burn fossil fuels at low efficiency and can
be a significant source of atmospheric pollutants. The services provided
by such peaking plant, include, but are not limited to,

[0004]
balancing differences in supply and demand at different times of the day
and at short notice,

[0005] providing electricity required to power the
auxiliary equipment required for restart of a generating asset in the
event of total network failure (black-start support),

[0006] network
reinforcement where parts of the electricity distribution network have a
shortfall in capacity during periods of high power demand,

[0007]
injecting power into the network to support the frequency of the grid
when demand for electricity increases rapidly.

[0008] In addition, the loss of power from the electricity distribution
network can result in significant economic loss to some consumers, such
as a data centre, or danger to personnel, for example in the event of a
power failure at a hospital. Such applications often utilise diesel
generators to provide standby electricity in the event of an interruption
to the supply of electricity from the distribution network. Replacement
of such diesel powered generators with a zero emissions device that uses
a fuel from a sustainable source would be of benefit.

[0009] There is a need for a device that can provide a similar service but
that uses a fuel that produces low or preferably zero atmospheric
pollution that originates from a sustainable source.

[0010] The present inventors have realised that there is potential to
generate electricity using the expansion of liquid air, liquid nitrogen
or cryogen to drive a turbine to generate electricity. Such a device
could provide a compact, reactive and environmentally clean solution to
the problems of balancing network supply with demand.

[0011] WO 2007/096656 discloses a cryogenic energy storage system which
exploits the temperature and phase differential between low temperature
liquid air, liquid nitrogen or cryogen, and ambient air, or waste heat,
to store energy at periods of low demand and/or excess production,
allowing this stored energy to be released later to generate electricity
during periods of high demand and/or constrained output. The system
comprises a means for liquefying air during periods of low electricity
demand, a means for storing the liquid air produced and an expansion
turbine for expanding the liquid air. The expansion turbine is connected
to a generator to generate electricity when required to meet shortfalls
between supply and demand. The target applications for the present
invention require a very low number of operating hours per year,
typically less than 500 and in the case of back-up power applications,
much less. It would be uneconomic to install a complete energy storage
system to service such applications due to the low utilisation of the air
liquefier equipment and relatively high cost of this equipment for such a
low level of utilisation.

[0012] PCT/BR2006/000177 discloses a device for generating power from
liquid air which utilises ambient heat to provide thermal energy for the
evaporation process. The inventors believe that this solution is
impractical as a very large area of heat transfer surface would be
required to prevent the build-up of excessive ice on the evaporator
during the evaporation of the cold cryogenic fluid.

SUMMARY OF THE INVENTION

[0013] The present invention provides a device and method as recited in
the claims, for the generation of zero emission electricity that can be
used to provide load balancing and emergency support to an electricity
distribution network, or back up power to a critical consumer such as a
hospital or data centre. The system uses a cryogenic fluid and a source
of low grade waste heat.

[0014] The present invention relates to electricity generation systems or
`cryogensets` and methods that use a cryogenic fluid such as liquid
nitrogen or liquid air and a source of low grade waste heat.

[0015] The present invention, referred to as the `cryogenset`, develops
the power recovery element of the prior art cited above to provide a
compact, clean, reactive and efficient electricity generation device and
method of generating electricity, which uses liquid air, liquid nitrogen
or cryogen as the working fluid. The working fluid is supplied by a
central plant that could service more than one cryogenset and other users
of cryogen and hence achieve a viable economic level of utilisation for
the liquefier.

[0016] The present invention utilises a cryogenic fluid, such as liquid
nitrogen or liquid air, and a source of low grade waste heat to power a
turbo-generator. The emissions from the device are either gaseous
nitrogen or gaseous air and present no environmental concerns. The
cryogenic fluid is manufactured in an industrial refrigeration or air
separation plant and supplied by tanker or pipeline to the cryogenset
preferably via a storage tank. The industrial refrigeration plant can be
powered by a sustainable source of energy, such as a wind turbine farm or
a solar plant, or by a low carbon source such as a nuclear power station.
In this way, the fuel consumed in the cryogenset is from a sustainable
source. FIG. 1 shows the cryogenset in relation to the heat source,
liquefier and end user.

[0017] The cryogenset is powered by the expansion of a high pressure gas
through an expansion turbine, which in turn drives a generator to produce
electricity. The high pressure gas is generated by a first compression of
a cryogenic fluid, typically air or nitrogen, in a pump, followed by
evaporation of the cryogenic fluid within an evaporator. In systems not
according to the present invention, evaporation of the cryogenic fluid
uses ambient heat alone and requires a large number of ambient
vaporisers. Such vaporisers are typically constructed from a finned tube
through which the cryogenic fluid passes. Heat is transferred through the
fins to the ambient environment. In such systems, the fins and tubes must
not be positioned too close together or excessive ice will build up on
the fins resulting in a degradation of performance and potentially
mechanical damage to the equipment due to the weight of the ice. This
problem is particularly relevant to, and is addressed by, the cryogenset
of the present invention, as a large quantity of cryogenic fluid must be
heated in a short space of time.

[0018] In the present invention, the cryogenic fluid is first evaporated
using the low pressure exhaust gases from the expansion turbine. The
consequentially high-pressure cold gas is further heated using a
superheater that takes thermal energy from a source of low grade waste
heat, such as a thermal power station or industrial process. The
combination of using the exhaust gas from the turbine and low grade waste
heat enables a much more compact, cost effective device to be designed
without the need for a large number of ambient vaporisers. Using the
exhaust gas to evaporate the cryogen removes the requirement for very low
temperature heat transfer fluids for this stage of the process, hence
improving the overall cost effectiveness of the system. Furthermore the
final exhaust gas from the system, at exit from the evaporator, is at
very low temperature, typically less than 10° C. higher than the
high pressure cryogenic liquid, and can be exhausted to atmosphere or
used in a co-located process that requires cold energy, such as a
refrigeration or air conditioning system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the present invention will now be described with
reference to the figures in which:

[0020] FIG. 1 shows the concept of the cryogenset of the present invention
in relation to a supply of cryogenic fluid from a refrigeration plant and
integration with a source of low grade waste heat;

[0021] FIG. 2 shows a cryogenset according to the present invention with a
single turbine stage;

[0022] FIG. 3 shows a second embodiment of a cryogenset according to the
present invention incorporating a second turbine stage to improve
efficiency;

[0023] FIG. 4 shows a third embodiment of a cryogenset according to the
present invention that utilises an additional heat transfer fluid
circuit; and

[0024] FIG. 5 shows a fourth, preferred embodiment of a cryogenset
according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] The concept of the cryogenset electricity generation system of the
present invention is shown in FIG. 1. Cryogenic liquid is manufactured at
the industrial refrigeration plant or air separation unit (ASU) 100 and
transferred by tanker or pipeline 110 to the storage container 120 local
to the cryogenset. When electricity is required to support the grid or
network or provide back-up supplies, the cryogenic liquid is released
from the storage container to the cryogenset 130, to generate electricity
to meet demand. The cryogenset is located close to a source of low grade
waste heat (typically 20° C. to 150° C.) 140, which is used
to improve the efficiency of the system as described in the following
embodiments. The power for the ASU 100 can be supplied from the
electricity distribution grid or network 150, from one or more of fossil
fuel, nuclear and renewable power stations and/or from a connection to a
renewable generation plant 160, such as a wind turbine.

[0026] The elements comprising different embodiments of the cryogenset 130
are now described with reference to FIGS. 2-4.

[0027] In a first embodiment of the present invention shown in FIG. 2,
cryogenic liquid is received from at least one storage tank 1 and
compressed to high pressure, typically greater than 70 bar but less than
200 bar, by at least one liquid pump 2. The high pressure liquid is then
evaporated using an evaporator 3 which is connected, on the heating side,
to the exhaust of an expansion turbine 10. The now gaseous high pressure
fluid is then further heated by another heat exchanger (referred to as
the superheater) 4 using heat, Q, from a source, or sources, of low grade
heat 20, such as a thermal power station or industrial process. The gas
is then expanded through the expansion turbine 10 to generate motive
power which in turn drives a generator 15 to produce electricity. The low
pressure exhaust gas from the turbine, which is at or slightly above
atmospheric pressure (typically 1 to 2 bar), is then returned to the
evaporator 3 to evaporate more of the incoming high pressure cryogenic
liquid. The final exhaust gas from the system is at a very low
temperature, typically less than 10° C. higher than the high
pressure cryogenic liquid, or -170° C. to -150° C., and can
be either exhausted to atmosphere or used in a co-located process that
requires cold energy such as a refrigeration or air conditioning system.

[0028] In a second embodiment of the invention as shown in FIG. 3, the
high pressure gas is expanded in two turbine stages 10, 11 to improve the
efficiency of the process. Although two stages 10, 11 are shown in FIG.
3, more than two turbine stages can be used. The efficiency is further
improved by reheating the part expanded gas between each turbine stage
using another heat exchanger, (referred to as a reheater) 5 and low grade
waste heat, Q', from at least one source of waste heat 20. In all other
respects the system of FIG. 3 is the same as that of FIG. 2. The source
of waste heat 20 used in the reheater 5 may be the same source or a
different source to that used in the superheater 4. The low pressure
exhaust gas from the final turbine stage 11 is then returned to the
evaporator 3 to evaporate the incoming high pressure cryogenic liquid.

[0029] When the source of low grade waste heat 20 is at a temperature
above 150° C., there are few cost effective heat transfer fluids
that can operate at a sufficiently high temperature and do not freeze at
the low temperatures encountered in the superheater 4. For example, many
low temperature hydrocarbon based heat transfer fluids can only operate
between -120° C. and 160° C. Examples of such low
temperature heat transfer fluids are those traded under the brand names
Dynalene MV, Paratherm CR. The heat transfer fluid would degrade
significantly if the heat source was, for example, the exhaust of a gas
turbine or diesel engine. High temperature heat transfer fluids that can
operate at temperatures over 200° C. will become very viscous and
even freeze if used below -30° C. An example of such a high
temperature heat transfer fluid is that traded under the brand name
Marlotherm LH.

[0030] For these cases, in a further embodiment of the invention as shown
in FIG. 4, an additional heat exchanger (referred to as the main heater)
30 may be added before the superheater 4 that enables two different heat
transfer fluids to be used, otherwise the system of FIG. 4 is the same as
that of FIG. 3. A first heat transfer fluid, which can operate at a low
temperature, is used to provide the first stage of heating in the main
heat exchanger 30. A second heat transfer fluid, which operates at a
higher temperature than the first heat transfer fluid, but may freeze if
used directly in the main heat exchanger 30, is used in the superheater 4
and reheater 5 between turbine stages 10, 11. The first heat transfer
fluid can be heated directly from one of the at least one sources of
waste heat 20 if the temperature is not too high, or otherwise indirectly
using the second heat transfer fluid (not shown in FIG. 4).

[0031] The present inventors have identified a number of power generation
processes that produce various grades of waste heat that could be used
with the cryogenset of the present invention. Some examples are
summarised in Table 1.

[0032] A preferred embodiment of the cryogenset is a two stage turbine
integrated with a source of waste heat of 200° C. to 250°
C., supplied from a waste incinerator, gas turbine or gas engine exhaust.
The inventors have found that two stage turbines are readily available
whereas more stages would require a bespoke design. In addition, current
cryogenic pumps are limited to 100 bar pressure and so the benefits of
more than two turbine stages are small without moving to a higher
pressure which would require the development of a new cryogenic pump. A
typical process flow diagram for the two stage configuration is shown in
FIG. 5 and typical pressures and temperatures for a 3 to 4 MW machine are
shown in Table 2. Both a high and low temperature heating loop are used,
as discussed with respect to FIG. 4, with two different heat transfer
media to ensure compatibility between the heat transfer fluids and the
heat exchanger surface temperatures. In the preferred embodiment, the
main heater inlet is -93° C. and high temperature heat transfer
fluids are more likely to be excessively viscous or freeze if used to
heat this heat exchanger. The reference numerals used in FIG. 5
correspond to the components and stages given in Table 2.

[0033] The present invention has been described above purely by way of
example. It should be noted, however, that modifications in detail can
been made within the scope of the invention as defined in the claims
appended hereto.